The use of r.f. current to effect the cutting and coagulation of body tissues has been known for many years, and comes under the broad description of electrosurgery. Two techniques to deliver the r.f. current to the tissues are in common usage today.
The first of these, monopolar electrosurgery, involves the use of an active (tissue treatment) electrode and a remote return (or neutral) electrode (or pad) placed on an external surface of the patient's body. Current flows from the active electrode, through the target site, and through any other tissue lying in the path between the target site and the return electrode. This arrangement introduces the potential for off-site burns, in other words tissue burns occurring at sites other than the target site. The medical literature includes references to numerous instances of capacitive coupling of the r.f. current to other instruments causing burns, direct coupling to tissue due to insulation failure, burns along the current path through the patient's body, and those occurring at the site of application of the return pad.
The second technique is known as bipolar electrosurgery. Bipolar electrosurgery involves the containment of current flow local to a target site by incorporating both the active and return electrodes close together, typically at the tip of the surgical instrument. This arrangement avoids the need for current flowing through the body to complete the electrical circuit, and hence eliminates the risks of off-site burns. The use of bipolar electrosurgery is, therefore, preferred where safety is of greatest concern, particularly when applying r.f. current close to vital structures, or when visualisation is limited such as during endoscopic surgery. As a result, bipolar coagulation or sealing of vessels during endoscopic surgery has become a cost-effective and easy to use alternative to the mechanical sealing of blood vessels using metal clips, staples or ligatures.
Normally, the electrosurgical instrument used for bipolar coagulation consists of a pair of forceps, in which each jaw of the forceps is an r.f. electrode. Depending on the size of the forceps, and hence the amount of tissue included in the circuit, the applied power can typically vary between 1W and 50W. The most significant problems encountered, when using conventional bipolar r.f. electrosurgery, are related to the distribution of energy throughout the tissue grasped between the forceps. As a result of these limitations, surgeons will commonly apply r.f. energy well beyond that necessary for effectively sealing a blood vessel, in theory to ensure complete sealing and to reduce the risk of bleeding when the vessel is subsequently divided. This leads to an excessive spread of the coagulation to adjacent tissues, and increases the risk of the forceps jaws becoming stuck to the tissue. This sticking can be sufficiently severe to cause coagulated tissue to be torn away when releasing the forceps, leading to damage of untreated areas of the vessel, and significant bleeding.
The industry standard for the coagulation output of a bipolar r.f. electrosurgery generator is a maximum power in the region of 50W-70W, with a specified load curve between 10 ohms and 1000 ohms. This power is normally delivered as a continuous, low crest factor waveform such as a sine wave. Peak voltages from such a generator can be as high as 1000V peak-to-peak. It has now been recognised, however, that lower voltages reduce the propensity to stick or carbonise the tissue when coagulating. Maximum voltages of up to 400V peak-to-peak are now more usually used in modern designs. The low impedance matching capability of this type of generator is limited, with maximum current delivery typically being in the region of 1.5A at full power.
Despite these advances, none of the known bipolar r.f. generators overcomes the problems of differential energy absorption within the tissue due to the variation in tissue impedance, the geometry of the forceps jaws, the presence of conductive fluids and tissue compression. As a result, coagulation is inevitably taken to the desiccation point, at which the tissue becomes dried out as fluids are boiled off, with an attendant elevation in the temperature of the forceps jaws. The cause of tissue sticking is the elevation in electrode temperature above 70-80° C. As this is more likely to occur because of the variables encountered during use, it is particularly likely to occur when the vessel to be treated is contained within the high impedance of a fatty layer, as is commonly encountered in vascular pedicles. The fatty layer effectively insulates the lower impedance vascular structure, so that incomplete scaling and excessive application are both more likely to occur.
For these reasons, it would be desirable to deliver bipolar r.f. electrosurgical energy in an improved way for coagulating tissues. It would be particularly desirable to provide more controlled absorption of energy throughout the tissue to be treated, largely irrespective of variables encountered during use, so that the problems of incomplete vessel sealing within fatty pedicles, tissue sticking and excessive thermal margin can be overcome. It would further be desirable to provide an improved bipolar r.f. electrosurgical output through an instrument such as that disclosed in U.S. Pat. No. 5,445,638 during endoscopic surgery.
Electrosurgical instruments have been proposed to resolve the problems of sticking. U.S. Pat. Nos. 3,685,518, 4,492,231 and 6,059,783 all describe methods of heat removal by constructing the electrodes of sufficient-thermal capacity, and/or by the use of thermally-conductive materials to dissipate heat. U.S. Pat. No. 5,885,281 describes the use of coatings to minimise the effects of sticking.
Impedance and temperature-based r.f. generator control is described in U.S. Pat. No. 5,496,312. Our U.S. Pat. No. 5,423,810 describes an impedance-controlled, bipolar cauterising output based on variations in the oscillator carrier frequency according to tissue impedance.
U.S. Pat. No. 6,033,399 discloses an electrosurgical generator capable of applying output power to surgical graspers in a manner such that the power level varies cyclically between low and high values in response to the changing impedance of the grasped tissue being treated, until the tissue is fully desiccated.
These techniques have had moderate success in terms of preventing sticking. One method of counteracting the negative temperature coefficient of resistance (NTCR) effect which tissue exhibits during coagulation is to introduce a positive temperature coefficient of resistance (PTCR) material, which new material is dominant. PTCR material produces the opposite effect to current hogging so that, instead of current hogging, the predominant effect would then be one of current sharing. Whilst it might be possible to coat the electrodes with a PTCR material, the material would dissipate heat and heat up the electrodes. Alternatively, a dielectric layer could be introduced with a positive temperature coefficient of impedance. This has the attraction of little or no heat dissipation, but unfortunately is very difficult to realise due to the lack of suitable materials.